Light sensor having undulating features for CMOS imager

ABSTRACT

Light sensors in an imager having sloped features including, but not limited to, hemispherical, v-shaped, or other sloped shapes. Light sensors having such a sloped feature can redirect incident light that is not absorbed by one portion of the photosensor to another portion of the photosensor for absorption there.

FIELD OF THE INVENTION

The present invention relates generally to the field of image sensors,and more particularly to solid state image sensors.

BACKGROUND OF THE INVENTION

Solid state image sensors are increasingly being used in a wide varietyof imaging applications as low cost imaging devices. One such sensor isa CMOS image sensor. A CMOS image sensor circuit includes a focal planearray of pixel cells, each one of the cells includes a photogate,photoconductor, or photodiode having an associated charge accumulationregion within a substrate for accumulating photo-generated charge. Eachpixel cell may include a transistor for transferring charge from thecharge accumulation region to a sensing node, and a transistor forresetting the sensing node to a predetermined charge level prior tocharge transference. The pixel cell may also include a source followertransistor for receiving and amplifying charge from the sensing node andan access transistor for controlling the readout of the cell contentsfrom the source follower transistor.

In a CMOS image sensor, the active elements of a pixel cell perform thenecessary functions of: (1) photon to charge conversion; (2)accumulation of image charge; (3) transfer of charge to the sensing nodeaccompanied by charge amplification; (4) resetting the sensing node to aknown state; (5) selection of a pixel for readout; and (6) output andamplification of a signal representing pixel charge from the sensingnode.

CMOS image sensors of the type discussed above are generally known asdiscussed, for example, in Nixon et al., “256×256 CMOS Active PixelSensor Camera-on-a-Chip,” IEEE Journal of Solid-State Circuits, Vol.31(12), pp. 2046-2050 (1996); and Mendis et al., “CMOS Active PixelImage Sensors,” IEEE Transactions on Electron Devices, Vol. 41(3), pp.452-453 (1994). See also U.S. Pat. Nos. 6,177,333 and 6,204,524, whichdescribe the operation of conventional CMOS image sensors and areassigned to Micron Technology, Inc., the contents of which areincorporated herein by reference.

An electrical schematic diagram of a conventional CMOS four-transistor(4T) pixel cell 10 is shown in FIG. 1. The CMOS pixel cell 10 generallycomprises a photosensor 14 for generating and collecting chargegenerated by light incident on the pixel cell 10, and a transfertransistor 17 for transferring photoelectric charges from thephotosensor 14 to a sensing node, typically a floating diffusion region5. The floating diffusion region 5 is electrically connected to the gateof an output source follower transistor 19. The pixel cell 10 alsoincludes a reset transistor 16 for resetting the floating diffusionregion 5 to a predetermined voltage V_(aa-pix); and a row selecttransistor 8 for outputting a reset signal V_(rst) and an image signalV_(sig) from the source follower transistor 19 to an output terminal inresponse to an address signal.

FIG. 2 is a cross-sectional view of a portion of the pixel cell 10 ofFIG. 1 showing the photosensor 14, transfer transistor 17 and resettransistor 16. The exemplary CMOS pixel cell 10 has a photosensor 14that may be formed as a pinned photodiode. The photodiode photosensor 14has a p-n-p construction comprising a p-type surface layer 13 and ann-type photodiode region 12 within a p-type active layer 11. Thephotosensor 14 is adjacent to and partially underneath the transfertransistor 17. The reset transistor 16 is on a side of the transfertransistor 17 opposite the photodiode photosensor 14. As shown in FIG.2, the reset transistor 16 includes a source/drain region 2. Thefloating diffusion region 5 is between the transfer and resettransistors 17, 16. An isolation trench 18 surrounds the pixel,isolating it from adjacent pixels.

In the CMOS pixel cell 10 depicted in FIGS. 1 and 2, electrons aregenerated by light incident on the photodiode photosensor 14 and arestored in the n-type photodiode region 12. These charges are transferredto the floating diffusion region 5 by the transfer transistor 17 whenthe transfer transistor 17 is activated. The source follower transistor19 produces an output signal based on the transferred charges. A maximumoutput signal is proportional to the number of electrons extracted fromthe photosensor 14. However, as seen in FIG. 2, a certain amount ofincident light is not absorbed by the photosensor 14, but rather, isreflected from its surface and lost. The loss of this incident lightdecreases responsivity, dynamic range and quantum efficiency of theimager.

Accordingly, it is desirable to have a photosensor that better capturesreflected incident light and directs it to the photosensor so the lightis absorbed and detected.

BRIEF SUMMARY OF THE INVENTION

Exemplary embodiments of the invention provide light sensors in animager having sloped features including, but not limited to,hemispherical, v-shaped, or other sloped shapes. Light sensors havingsuch a sloped feature can redirect unabsorbed incident light such thatthe light reflected from one portion of a light sensor is directed toanother location on the same light sensor. This improves the amount ofincident light which is absorbed.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the various embodiments of the inventionwill be more readily understood from the following detailed descriptionof the invention which is provided in connection with the accompanyingdrawings:

FIG. 1 is a schematic diagram of a convention four-transistor (4T) pixelcell;

FIG. 2 is a cross-sectional view of a fabricated portion of the FIG. 1pixel cell;

FIG. 3 is a cross-sectional view of a fabricated pixel cell inaccordance with an exemplary embodiment the present invention;

FIG. 4 a is an expanded cross-sectional view of region A of FIG. 3;

FIG. 4 b is an expanded cross-sectional view of another exemplaryembodiment of the present invention;

FIG. 4 c is an expanded cross-sectional view of another exemplaryembodiment of the present invention;

FIG. 5 a is a three-dimensional view of one configuration of theembodiment of FIG. 4 a;

FIG. 5 b is a three-dimensional view of another configuration of theembodiment of FIG. 4 a;

FIG. 6 a is a plan view of a pixel having the configuration of FIG. 5 b;

FIG. 6 b is a plan view of an array of pixels of FIG. 6 a;

FIG. 7 is a cross-sectional view of the present invention at an earlystage of fabrication;

FIG. 8 is a cross-sectional view of the present invention at a stage offabrication subsequent to FIG. 7;

FIG. 9 a is an expanded cross-sectional view of the present invention ata stage of fabrication subsequent to FIG. 8;

FIG. 9 b is an expanded cross-sectional view of the present invention ata stage of fabrication subsequent to FIG. 9 b;

FIG. 9 c is an expanded cross-sectional view of the present invention ata stage of fabrication subsequent to FIG. 9 b;

FIG. 10 is a block diagram of an imaging device according to the presentinvention; and

FIG. 11 is a block diagram of a processing system including the imagingdevice of FIG. 10.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof and illustrate specificexemplary embodiments by which the invention may be practiced. It shouldbe understood that like reference-numerals represent like-elementsthroughout the drawings. These embodiments are described in sufficientdetail to enable those skilled in the art to practice the invention, andit is to be understood that other embodiments may be utilized, and thatstructural, logical and electrical changes may be made without departingfrom the spirit and scope of the present invention.

The term “substrate” is to be understood as includingsilicon-on-insulator (SOI) or silicon-on-sapphire (SOS) technology,doped and undoped semiconductors, epitaxial layers of silicon supportedby a base semiconductor foundation, and other semiconductor structures.Furthermore, when reference is made to a “substrate” in the followingdescription, previous process steps may have been utilized to formregions or junctions in the base semiconductor structure or foundation.In addition, the semiconductor need not be silicon-based, but could bebased on silicon-germanium, germanium, gallium arsenide, or othersemiconductor material, for example.

The term “pixel” or “pixel cell” refers to a picture element unit cellcontaining a photosensor and transistors for converting light radiationto an electrical signal. For purposes of illustration, a representativepixel is illustrated in the figures and description herein and,typically, fabrication of all pixels in an imager pixel array willproceed simultaneously in a similar fashion. Moreover, while afour-transistor pixel cell is described, the invention is not limited tosuch an embodiment. The invention may be employed with any typicalelectrical pixel architecture, such as a two-transistor,three-transistor, five- or more transistor pixel cells. The invention isalso not limited to CMOS pixels and may be employed in pixels of solidstate imagers, a CCD imager being just one example of another type ofsolid state imager.

The term “light” refers to electromagnetic radiation that can produce avisual sensation (visible light) as well as electromagnetic radiationoutside of the visible spectrum. In general, light as used herein is notlimited to visible radiation, but refers more broadly to the entireelectromagnetic spectrum, particularly electromagnetic radiation thatcan be converted by a solid state photosensor into a useful signal.

Referring now to the drawings, where like elements are designated bylike reference numerals, FIG. 3 illustrates a cross-section of a pixelcell 20 according to an exemplary embodiment, which is electricallyschematically similar to the pixel cell 10 of FIG. 1. Thecross-sectional view of pixel cell 20 shows a photodiode photosensor 24,transfer transistor 27 and reset transistor 26. Photodiode photosensor24 is formed as a pinned photodiode having a p-n-p constructioncomprising a p-type surface layer 23 and an n-type photodiode region 22within a p-type active layer 21. The photodiode photosensor 24 isadjacent to and partially underneath the transfer transistor 27. Thereset transistor 26 is on a side of the transfer transistor 27 oppositethe photodiode photosensor 24. As shown in FIG. 3, the reset transistor26 includes a source/drain region 29. The floating diffusion region 25is between the transfer and reset transistors 27, 26. Isolation trenches28 are formed in the substrate. It should be noted that the bottom ofthe isolation trench 28 is deeper than the lowest top surface of thesilicon substrate by a depth of d, which is preferably greater than 2000Å. Although the photodiode photosensor 24 is shown as a P—N—Pphotodiode, it can also be formed as an N—P—N photodiode as would beunderstood by those skilled in the art.

The photodiode photosensor 24 does not have a planar upper surface. Ithas instead an upper surface profile which provides slanted or curvedsidewalls capable of directing light reflected off one portion of thephotosensor to another portion for the photosensor for capture. In theFIG. 3 embodiment, the upper surface has a series of undulating featuresforming side walls 24 a that form a tapered peak 24 b, shown in greaterdetail in FIG. 4 a, which is an expanded cross-section view of region Aof FIG. 3. Region A, as shown in FIG. 4 a, has a v-shapedcross-sectional profile; however, other cross-sectional profiles, e.g.,more rounded cross-sectionals and a u-shaped peak, would also work. Someof the incident light is absorbed by the photosensor 24, however, someof the incident light is reflected off the surface of the p-type surfacelayer 23. With the v-shaped configuration illustrated in FIG. 3, thelight that is reflected off the surface of the p-type surface layer 23is redirected to another location on the photosensor 24 to have anotherchance at being absorbed into the photosensor 24.

If light is not absorbed at that location, it may be reflected and againredirected to another location on the photosensor 24 to have yet anotherchance at being absorbed into the photosensor 24. While multipleredirection of reflected light may occur in any embodiment, it isillustrated in the embodiments of FIGS. 4 b and 4 c, for example,discussed below.

FIG. 4 b is an expanded cross-section view of another embodiment,wherein the photosensor 24′ of a pixel cell (e.g., pixel cell 20 of FIG.3) has curved sloped side walls 24′a with a u-shaped trench 24′cconfiguration. While FIG. 4 b shows a flat-bottomed trench 24′c, it mayalso have a rounded bottom or v-shaped bottom. Again, sidewalls 24′a areprovided in the upper surface profile of photosensor 24′ capable ofredirecting reflected light to from one portion of the photosensor toanother.

FIG. 4 c is an expanded cross-sectional view of another embodiment,wherein the photosensor 24″ of a pixel cell (e.g., pixel cell 20 of FIG.3) has sloped features with a u-shaped trench configuration havingpointed tips, or peaks. The various shapes of the tips and trenches maybe obtained by selecting different methods of masking and/or etching, aswill be described in more detail below. While FIG. 4 c shows aflat-bottomed trench, it may also have a rounded bottom or a v-shapedbottom.

Referring back to the embodiment of FIGS. 3 and 4 a, the v-shapedcross-section may have a cone shape, in three-dimensions, as shown inFIG. 5 a. Generally, the cone may have a height h of approximately 2.0μm and a width or diameter w of approximately 2.0 μm. However, thediameter of the cones and/or the spacing between them may be selected tooptimize capture of specific colors having different wavelengths. Forexample, the pitch p, or the distance between the peaks of the cones ispreferably not less than approximately a quarter of the wavelength ofthe desired wavelength of light, i.e., p≧¼ λ.

Alternatively, the v-shaped cross-section may have a prism shape, asshown in FIG. 5 b. Like the cone-shaped configuration illustrated inFIG. 5 a, the prisms of FIG. 5 b may have a height h of approximately2.0 μm and a width w of approximately 2.0 μm. However, the dimensions ofthe prisms and/or the spacing between them may be selected to optimizecapture of specific colors having different wavelengths. For example,the pitch p, or the distance between the peaks of the prism ispreferably not less than approximately a quarter of the wavelength ofthe desired wavelength of captured light, i.e., p≧¼ λ.

Generally, a photosensor according to the present invention has agreater signal-to-noise ratio than a prior art photosensor. However,there may still be some scatter due to a minimal amount of incidentlight that is never absorbed by the photosensor of the presentinvention. For instance, a photosensor having a cone-shapedconfiguration has a greater surface area than a photosensor having aprism-shaped configuration, however a cone-shaped configuration may havea tendency to scatter a greater amount of light to neighboring pixels.Therefore, dimensions and spacing of both cone- and prism-shapedconfigurations may be selected to increase surface area for photoncapture and minimize scatter.

In the case of a prism-shaped configuration, reflecting surfaces arepreferably located so that any scattered light will go to neighboringpixels that are not being read at the same time, thereby minimizingoptical cross-talk. The layout of the prism-shaped configuration isillustrated in plan view in FIG. 6 a, where the cross-section of FIG. 3is taken along line X-X of FIG. 6 a. The reflecting surfaces 30 arealigned such that the scattering occurs in a direction that ishorizontal to the direction that the image will be viewed, asillustrated in the pixel array of FIG. 6 b, since the rows of pixels aretypically read out as horizontal stripes. Such scattering is preferablesince the human eye will not detect color cross-talk in the verticaldirection as easily as in the horizontal direction.

Although the photodiode photosensor 24 of FIG. 3 is shown to have aseries of sloped features, it should be noted that the invention is notlimited to such an embodiment. As discussed above, a photosensor havinga pitch greater than ¼ the wavelength of light is suitable. In addition,a photosensor according the present invention may have a single trench,rather than a series of sloped features.

The formation of the pixel cell 20 of the invention is now described.The earlier processing steps of the pixel 20 include any known stepsthat form transfer transistor 27, reset transistor 26, floatingdiffusion region 5, source/drain region 29, a p-type active layer 21 andan n-type photodiode region 22, as shown in FIG. 7. The isolation trench28 should be formed to a depth of at least “d” deeper than theanticipated lowest top surface of the substrate, shown in FIG. 7 as ahashed line. The depth d is preferably greater than 2000 Å. A sourcefollower transistor and a row select transistor are also formed (notshown). The substrate is then masked (not shown) and cones or prisms areetched out of n-type photodiode region 22, as shown in FIG. 8. Themasking pattern and dry etch may be performed as described in U.S. Pat.No. 6,416,376 to Wilson and U.S. Pat. No. 5,391,259 to Cathey et al.,which are hereby incorporated by reference. These masking and etchingmethods are used to form a substantially uniform array of sharp tips inthe substrate by dry etching. The substrate may also be cleaned afterthe etching step, by washing in a wash of deionized water or bufferedoxide etch, as described in the '376 patent.

Some defects in the silicon may result from the etching process. Thesedefects may result in increased dark current from the photosensitiveregion. Therefore, the defects may be cured by techniques typically usedon CMOS transistor gates. A first technique is illustrated in theexpanded cross-section view of FIGS. 9 a-9 c. This first techniqueincludes under-etching the cone or prism features from the final desiredshape 22 a (indicated in hashed lines) and depth by approximately 150 Åto produce a surface shape 22 b, as shown in FIG. 9 a. A sacrificialSiO₂ layer 32 is grown over the surface shape 22 b of the features, asshown in FIG. 9 b. A standard oxide process is performed, oxidizing thesilicon such that the SiO₂ layer 32 is consumed by the defects and thedesired depth and shape 22 a of the surface of the features is obtained,as shown in FIG. 9 c. For example, if a 200 Å layer of SiO₂ is depositedover the etched features of the silicon and undergoes a dry O₂ flow at800° C., approximately 100 Å of the silicon is consumed.

A second technique includes treating the silicon with H₂. The H₂treatment heals the dangling bonds of the defective silicon and allowsthe silicon to migrate to local energy minima at the surface of thesilicon. Both the first and second techniques may be performed tominimize defects.

Referring back to FIG. 8, The final p-type surface layer 23 of FIG. 3 isformed by performing a shallow p-type doping implant over the n-typephotodiode region 22. Formation of the remainder of the pixel 20 isperformed using any known processing steps.

FIG. 10 illustrates an exemplary imaging device 200 that may utilizepixels having photosensors constructed in accordance with the invention.The imaging device 200 has an imager pixel array 100 comprising aplurality of pixels with photosensors constructed as described above.Row lines are selectively activated by a row driver 202 in response torow address decoder 203. A column driver 204 and column address decoder205 are also included in the imaging device 200. The imaging device 200is operated by the timing and control circuit 206, which controls theaddress decoders 203, 205. The control circuit 206 also controls the rowand column driver circuitry 202, 204.

A sample and hold circuit 207 associated with the column driver 204reads a pixel reset signal Vrst and a pixel image signal Vsig forselected pixels. A differential signal (Vrst−Vsig) is produced bydifferential amplifier 208 for each pixel and is digitized byanalog-to-digital converter 209 (ADC). The analog-to-digital converter209 supplies the digitized pixel signals to an image processor 210 whichforms and outputs a digital image.

FIG. 11 shows a typical processor system which includes the imagingdevice 200 (FIG. 10) of the invention. The processor system 900 isexemplary of a system having digital circuits that could include imagesensor devices. Without being limiting, such a system could include acomputer system, still or video camera system, scanner, machine vision,vehicle navigation, video phone, surveillance system, auto focus system,star tracker system, motion detection system, image stabilizationsystem, and other imaging systems.

The processor-based system 900, for example a camera system, maycomprise a central processing unit (CPU) 995, such as a microprocessor,that communicates with an input/output (I/O) device 991 over a bus 993.Imaging device 200 also communicates with the CPU 995 over bus 993. Theprocessor system 900 also includes random access memory (RAM) 992, andcan include removable memory 994, such as flash memory, which alsocommunicate with CPU 995 over the bus 993. Imaging device 200 may becombined with a processor, such as a CPU, digital signal processor, ormicroprocessor, with or without memory storage on a single integratedcircuit or on a different chip than the processor.

While the invention has been described in detail in connection withexemplary embodiments known at the time, it should be readily understoodthat the invention is not limited to such disclosed embodiments. Rather,the invention can be modified to incorporate any number of variations,alterations, substitutions or equivalent arrangements not heretoforedescribed, but which are commensurate with the spirit and scope of theinvention. For example, while the invention has been described asforming a photodiode photosensor having a p-n-p construction, an n-p-nor other construction may be used to form the photosensor. Accordingly,the invention is not to be seen as limited by the foregoing description,but is only limited by the scope of the appended claims.

1. An imaging device photosensor comprising: a photosensitive region formed in a substrate; and at least one sloped feature formed in said photosensitive region for redirecting reflected light from one portion of said photosensitive region to another portion of said photosensitive region.
 2. The photosensor of claim 1, wherein said at least one sloped feature has a cone-shaped configuration.
 3. The photosensor of claim 1, wherein said at least one sloped feature has a prism-shaped configuration.
 4. The photosensor of claim 1, further comprising a plurality of sloped features formed in said photosensitive region.
 5. The photosensor of claim 4, wherein the distance between any two sloped features is greater than approximately ¼ the wavelength of light to be absorbed by said photosensor.
 6. The photosensor of claim 5, wherein a cross-section of any two adjacent sloped features form a v-shape.
 7. The photosensor of claim 5, wherein a cross-section of any two adjacent sloped features form a hemispherical shape.
 8. The photosensor of claim 5, wherein a cross-section of any two adjacent sloped features have a flat area between them.
 9. The photosensor of claim 1, wherein said photosensor is a photodiode sensor.
 10. A photosensor for use in an imaging device, said photosensor comprising: a first doped layer of a first conductivity type formed in a substrate; a second doped layer of a second conductivity type formed in said substrate over said first doped layer, wherein said second doped layer has at least one sloped feature having a sloped upper surface for redirecting reflected light from one portion of said photosensitive region to another portion of said photosensitive region; a third doped layer of a first conductivity type formed in said upper surface of said at least one sloped feature.
 11. The photosensor of claim 10, wherein said at least one sloped feature has a cone-shaped configuration.
 12. The photosensor of claim 10, wherein said at least one sloped feature has a prism-shaped configuration.
 13. The photosensor of claim 10, further comprising a plurality of sloped features formed in said second doped layer in said substrate.
 14. The photosensor of claim 10, wherein said first conductivity type is p-type.
 15. The photosensor of claim 14, wherein said second conductivity type is n-type.
 16. The photosensor of claim 13, wherein the distance between any two adjacent sloped features is greater than approximately ¼ the wavelength of light to be absorbed by said photosensor.
 17. The photosensor of claim 16, wherein a cross-section of any two adjacent sloped features form a v-shape.
 18. The photosensor of claim 16, wherein a cross-section of any two adjacent sloped features form a hemispherical shape.
 19. The photosensor of claim 16, wherein a cross-section of any two adjacent sloped features has a flat area between them.
 20. An imager device comprising: an array of pixels, each comprising: a photosensitive region formed in a substrate; and at least one sloped feature formed in said photosensitive region for redirecting reflected light from one portion of said photosensitive region to another portion of said photosensitive region.
 21. The imager device of claim 20, wherein said at least one sloped feature comprises photosensitive materials.
 22. The imager device of claim 20, wherein said at least one sloped feature has a cone-shaped configuration.
 23. The imager device of claim 20, wherein said at least one sloped feature has a prism-shaped configuration.
 24. The imager device of claim 20, further comprising a plurality of sloped features formed in said photosensitive region.
 25. The imager device of claim 24, wherein the distance between any two sloped features is greater than approximately ¼ the wavelength of light to be absorbed by said photosensor.
 26. The imager device of claim 25, wherein a cross-section of any two adjacent sloped features form a v-shape.
 27. The imager device of claim 25, wherein a cross-section of any two adjacent sloped features form a hemispherical shape.
 28. The imager device of claim 25, wherein a cross-section of any two adjacent sloped features have a flat area between them.
 29. The imager device of claim 20, wherein said photosensor is a photodiode sensor.
 30. An image processor comprising: a processor; a pixel array coupled to said processor comprising: a photosensor having a photosensitive area, wherein said photosensitive area has at least one sloped feature on a top surface for redirecting reflected light from one portion of said photosensitive region to another portion of said photosensitive region.
 31. The image processor of claim 30, further comprising a plurality of sloped features on said top surface.
 32. The image processor of claim 30, wherein said at least one sloped feature has a cone-shaped configuration.
 33. The image processor of claim 30, wherein said at least one sloped feature has a prism-shaped configuration.
 34. A method of forming a photosensor, comprising the steps of: providing a semiconductor substrate having a first doped layer of a first conductivity type; forming a second doped region of a second conductivity type to define a photosensitive area; forming at least one sloped feature in said second doped region for redirecting reflected light from one portion of said photosensitive region to another portion of said photosensitive region; and forming a third doped region of said first conductivity type over said second doped region.
 35. The method of claim 34, wherein said step of forming at least one sloped feature in said second doped region further comprises growing a sacrificial SiO₂ layer and performing an oxidation process.
 36. The method of claim 34, wherein said step of forming at least one sloped feature in said second doped region further comprises treating said substrate with an H₂ treatment after said sloped feature is formed.
 37. The method of claim 34, wherein said first conductivity type is p-type and said second conductivity type is n-type.
 38. The method of claim 34, wherein said step of forming at least one sloped feature in said second doped region comprises forming at least one cone-shaped feature.
 39. The method of claim 34, wherein said step of forming at least one sloped feature in said second doped region comprises forming at least one prism-shaped feature.
 40. The method of claim 34, wherein said step of forming at least one sloped feature in said second doped region further comprises forming a plurality of sloped features. 